Field
The present invention relates to a method for measuring a flow rate (flow rate measurement method) in a thermal type mass flow meter (thermal mass flow meter), a thermal type mass flow meter which uses the method, and a thermal type mass flow control device (thermal mass flow controller) which uses the thermal type mass flow meter.
Background
Mass flow meters have been widely used for the purpose of measuring a mass flow rate of a process gas supplied into a chamber in a manufacturing process of a semiconductor, for example. In addition, a mass flow meter is not only used independently as mentioned above, but is also used as a part which constitutes a mass flow control device (mass flow controller) with other members, such as a flow control valve and a control circuit. Although there are various kinds of mass flow meters in the art, a thermal type mass flow meter is used widely among them since a mass flow rate of a fluid (for example, a gas and a liquid) can be measured accurately with a relatively simple configuration.
Generally, a thermal type mass flow meter is constituted by a passage through which a fluid flows, a bypass which is prepared in the middle of the passage, a sensor tube which branches from the passage on an upstream side of the bypass and joins the passage again on a downstream side of the bypass, a pair of sensor wires wound around the sensor tube and a sensor circuit which comprises a bridge circuit including the sensor wires and other resistive elements (see for example, Japanese Patent Application Laid-Open “kokai” No. 2009-192220 official report). The bypass has a flow resistance against a fluid, and is constituted so that a constant proportion of a fluid which flows through the passage branches into the sensor tube.
In the above-mentioned configuration, when the pair of sensor wires is made to generate heat by applying a predetermined electric voltage (or a predetermined electric current) thereto, heat generated from the sensor wires is removed (drawn) by a fluid which flows through the sensor tube. As a result, the fluid which flows through the sensor tube is heated. In this case, the sensor wire on the upstream side has its heat removed by the fluid which has not yet been heated. On the other hand, the sensor wire on the downstream side has its heat removed by the fluid which has been already heated with the sensor wire on the upstream side. For this reason, the heat removed from the sensor wire on the upstream side is larger than the heat removed from the sensor wire on the downstream side. As a result, temperature of the sensor wire on the upstream side becomes lower than temperature of the sensor wire on the downstream side. For this reason, an electrical resistance value of the sensor wire on the upstream side becomes lower than an electrical resistance value of the sensor wire on the downstream side. A difference in the electrical resistance value resulting from thus produced temperature difference between the sensor wire on the upstream side and the sensor wire on the downstream side becomes larger, as a mass flow rate of the fluid which flows through the sensor tube becomes larger.
A change of the difference in the electrical resistance value between the sensor wire on the upstream side and the sensor wire on the downstream side according to a mass flow rate of the fluid as mentioned above can be detected by using a bridge circuit, etc., for example. Furthermore, based on thus detected difference in change of the electrical resistance value of the sensor wire, a mass flow rate of the fluid which flows through the sensor tube can be obtained, and a mass flow rate of the fluid which flows through the passage can be obtained based on the mass flow rate of the fluid which flows through the sensor tube (will be mentioned later in detail). In this specification, a portion including a sensor tube and sensor wires of a thermal type mass flow meter is referred to as a “flow sensor.”
As a material for a sensor tube, a material which has an excellent corrosion resistance and an excellent mechanical strength is desirable, and metal (namely, conductor), such as a stainless steel, is used generally. On the other hand, as a matter of course, a conductor is used as a material for a sensor wire. Namely, in general, both the materials for a sensor tube and a sensor wire are conductors. Therefore, it is general in a flow sensor that, for the purpose of prevention of electric continuity between a sensor tube and sensor wires and electric continuity between sensor wires and fixation of sensor wires to a sensor tube, etc., a covering layer formed of an insulation material, such as resin, for example, is disposed in the surroundings of a portion of the sensor tube where the sensor wires are wound and in the surroundings of the sensor wires (will be mentioned later in detail).
In addition, in order to measure a mass flow rate by a flow sensor, as mentioned above, heat generated from sensor wires due to electrification needs to be removed by a sensor tube and a fluid which flows through the sensor tube. Therefore, it is desirable that at least a covering layer which intervenes between sensor wires and a sensor tube has good thermal conductivity.